Astrophotography requires long exposure times, which presuppose that the telescope is accurately aligned by means of a motor drive (for example, step motors). A corresponding controller makes it possible to orient the telescope to an observation object and to track it.
Equatorial mounts can be pivoted for this purpose in the right ascension axis and declination axis. The polar axis must be oriented parallel to the axis of the earth in order to counteract a constant velocity (sidereal) in the driven right ascension axis of the rotation of the earth. Since this placement is very error-prone, the two axes are increasingly equipped with motors and driven to compensate for the error of incorrect placement.
Telescopes with larger focal lengths (>300 mm) are often used for astrophotography and longer (>5 min) exposure times are selected in order to take images. In combination with current cameras and their image sensors with small pixels (<10 μm), very high accuracies (path deviation of max.+−1″ in 10 minutes) are necessary during tracking. Even with perfect alignment of the mount, the movement of the axes with the engines and gearboxes available today would be too inaccurate to meet these requirements. For this reason, the drives must be regulated as a function of the measured movement of the axes.
Up till now, so-called “autoguiders” have been increasingly used for measuring the movement of the axes, wherein these systems require a second image sensor that observes a guide star and determines its position. When a deviation from the target position occurs, a correction for the movement of the axes is calculated and sent to the axis controller. This is a control circuit, which has to be correspondingly parameterized with great effort. Autoguiders exist in different designs, either as an additional image sensor in special cameras, having their own optics mounted on the telescope or mounted in the beam path of the main optics with the aid of an off axis guider. The disadvantage of the autoguider is that the complexity of the entire system increases and operability is made more difficult, because, among other things, a control system, depending on the optics and mount, must be parameterized. Furthermore, power consumption is increased by the second camera and, if necessary, by a second dew protection for the guide scope.
Another known method is the creation of a pointing model. With the aid of the mounted optics and camera, the position of celestial objects is then determined exactly. After measuring the position of several objects, a model is calculated with which such a mount can be accurately positioned and adjusted. The disadvantage of this method is that the models lose their validity when the mount changes its position. The smallest changes in the position or orientation between the mount and the celestial object are sufficient for this purpose. Such changes can occur on the stand, the telescope and the camera. This is especially the case in mobile operation (e.g., by sinking of the stand or by thermal expansion in the mount).
A pointing model for the mount is provided in US 2013/0 265 639 A1. This is done before the actual taking of the main images. The main sensor does not function as an adjustment control sensor, but as a sensor for the one-time creation of the pointing model. US 2013/0 265 639 A1 also describes the calibration of rotary encoders by means of astronomical methods so that lower cost rotary encoders can be used. In this case, the actual adjustment takes place via a previously determined pointing model and does not use the main sensor for adjustment control.
GB 2 485 596 A describes a system that integrates an adjustment control camera and optics into the mount (self-guiding mount). An additional camera and optics are necessary for the actual main image, wherein, however, no main image is taken with the adjustment control camera.
A system is also described in US 2014/0 085 717 A1, which integrates an adjustment control camera and an optical system in the mount. In addition, a further imaging sensor and an optical system are provided, with which an exact measurement of the position in the sky is possible. An additional camera and optics are necessary for the actual main image, wherein no main image is created with the sensors and optics used.